Defining Baseline Model for Bridge Analysis and Design

In current AASHTO design practices, bridges are designed on an elemental basis. AASHTO code specifies that each structural element is to be designed for the loads it will experience during the life of the bridge. The calculated loads are enveloped to produce maximum loads. This approach, elemental design, is structurally sound, in general conservative, and has resulted in the design and construction of safe and reliable bridges. Designers use a matrix approach to evaluate a set of what are essentially initial-state load conditions. This paper describes development of a "baseline" bridge model and suggests certain modifications to the traditional bridge design process to take advantage of modern computing capabilities to create a refined baseline model. The baseline model is an analytical model which accounts for the full system behavior of the structure, as opposed to the elemental approach which envelopes loads and conditions to evaluate separate components of the bridge structure. The baseline model is a three-dimensional structural model employing finite element software. A sample baseline model is developed and evaluated for a simple span steel composite concrete stringer bridge. Advantages and challenges for application of the baseline model, as part of a modified overall bridge design process, are presented and discussed.